In many scrambled cable systems today, illegal cable black boxes are used to present a viewable or stable picture. These cable black boxes are effective in neutralizing sync suppressed scrambled video systems. It has also been found that some video scrambling systems using vertical sync suppression and/or horizontal sync modulation are also vulnerable to these cable black boxes. By using relatively simple circuitry with at least one manual control, the operator with a cable black box adjusts the control until a viewable picture is seen.
Referring to the FIG. 1, in those black boxes with at least one adjustment, the video level is adjusted via an amplifier 12, and then supplied to a low pass filter 14 and a comparator 16. The comparator then generates a vertical timing signal V reset for a television sync generator circuit 18 which also receives a horizontal timing signal from a filter/slicer circuit 20. Once a reliable vertical reset pulse is established (output) by the comparator 16, the sync generator circuit 18 delivers composite sync for a viewable picture via a switcher 22. Even with sync suppressed video signals, the comparator 16 picks out the normally blanked lines in the vertical interval and generates a suitable vertical reset signal.
In yet another type of cable black box, the color burst of the video signal is used for illegal decoding. In this case, the lack of color burst in some lines of the vertical, blanking interval identifies the vertical sync area. Thus, for a cable black box such as shown in FIG. 2, the color burst is relied upon to provide the illegal decoding process. More specifically, the lack of burst in the vertical blanking interval (VBI) allows for the illegal generation of a reliable vertical rate reset signal. The fact is that in most video signals, scrambled or not, burst is not present for about 9 lines in the VBI. Thus in FIG. 2, the scrambled video signal is bandpass filtered in a filter 24 to provide a burst envelope signal which is supplied to a detection amplifier 26 and a phase lock loop (PLL) circuit 28. The detection amplifier senses the burst envelope. A one shot 30 triggers off the burst envelope leading edge and supplies a pulse which extends into the next video line (e.g., is 50 microseconds) and thus is referenced to the color burst, not the video signal. A one shot 32 provides a short pulse (of about 2 to 3 microseconds) as a burst gate to the PLL circuit 28, which also receives the burst envelope signal. The PLL circuit 28 supplies a burst signal and a clock reference to a sync regenerator circuit 36. A retriggerable one shot 34 is triggered by the one shot 30 pulse and provides a pulse which is slightly longer than one video line, and which thus extends through the active video field so that the one shot 30 no longer sees the burst envelope. At this point the retriggerable one shot 34 turns off during most of the VBI (e.g., for about 20 video lines). Thus, the output of one shot 34 comprises a regenerated vertical rate pulse, which is supplied to the sync regenerator circuit 36. The latter circuit 36 supplies new sync/burst signals as well as an insert control signal to a switch circuit 38. The switch circuit 38 also receives the scrambled video signal and, in response to the insert control signal, inserts the new sync/burst signals to descramble the signal sufficiently to provide a viewable video signal to an unauthorized user via an amplifier 40.